Device for electrical discharge piercing of holes in components

ABSTRACT

A device for electrical discharge piercing of holes in components comprises a tool electrode mounted in an electrode holder (18) and made as a rod (26) enclosed, with a clearance, in a sheath (27) and somewhat projecting therefrom, and a contrivance for traversing, locking and wear compensation of the tool electrode. The aforesaid contrivance comprises a driving carriage (11) and a driven carriage (12), both being mechanically interlinked and set in parallel to each other, each of the carriages carrying a clamp adapted for alternately locking the rod (26) of the tool electrode, the clamp being made as a movable jaw (29,30) and a fixed jaw (45,46), while the housing of each carriage (11, 12) has through center-aligning holes situated at the level of the gap between the movable jaw (29,30) and the fixed jaw (45,46) and arranged coaxially with a channel (23) made in a union locked in place in the bottom portion of the electrode holder (18), the tool electrode rod (26) being axially traversable in the channel ( 23), the sheath of the tool electrode rod being fixed in place on the electrode holder union coaxially with the tool electrode rod, and the electrode holder is fixed in place on the driving carriage.

FIELD OF THE INVENTION

The present invention relates generally to electrical dischargemachining of metals and more specifically, to a deivce for hole piercingin various components.

BACKGROUND ART

Known in the present state of the art is a device for electricaldischarge piercing of holes by means of a tool electrode (U.S. Pat. No.4,543,460). The known device comprises an electrode holder which is infact a guide element through whose bore an electrode is passed with apossibility to traverse vertically lengthwise its longitudinal axisunder the action of an electrode feed drive. The drive is powered from anumerical control device whose compound signals (pulses) control theamount of spark gap between the electrode working end and the workpiece,as well as the rate of the electrode feed depthwise the workpiece.Besides, the electrode feed drive responds to electric signals deliveredfrom the spark gap so as to adjust the position of the electrode workingend with respect to the workpiece and to maintain constant spark gap.The amount of the spark gap is to be maintained constant also with thepurpose of compensating for spark-erosion wear on the electrode. Theelectrode feed drive may perform also an additional function, i.e., torotate the electrode about its longitudinal axis and impart oscillatingmotion to the electrode lengthwise its longitudinal axis.

Electrical discharge machining medium or working fluid is supplied underpressure from the tank along a pipe to the bore of the guide element andfurther on into the spark gap. In addition, the device is provided witha block which effects control over disposal of the spark-erosion wasteproducts from the working gap so that concentration of said wasteproducts be maintained somewhat above the preset level, thus preventingformation of electric discharge pulses from destabilization and makingfor stable rate of electrode feed and the entire workpiece machiningprocess.

The block is situated in close proximity to the workpiece surface with asmall clearance between the electrode and the workpiece; it has a centrehole for the electrode to pass through, a clearance being left betweenthe hole faces and the electrode. An annular recess is provided in thetop portion of said block, adapted to receive the bottom portion of thecylinder-shaped guide element carrying the electrode, both the formerand the latter being held together. The centre hole in the blockcommunicates with the bore of the guide element, wherein the electrodetop portion is accommodated. The bottom block portion arrangedlengthwise the workpiece surface, establishes a uniform clearance wtihthe workpiece surface. The block is vertically traversable from its owndrive, which is also controlled by command signals delivered from anumerical control device so as to cause the block to alter its verticalposition with the purpose of maintaining the amount of clearance betweenthe block lower surface and the workpiece surface constant. The drive isalso capable of responding to a change in the amount of working (spark)gap and hence in the machining conditions.

However, the aforementioned electrode construction featuring the feeddrive and compensation for the consumable electrode portion inconjunction with a working liquid recirculation in the machining zonerenders the known device unsuitable for piercing deep or long holeswhose diameter would be comparable with the electrode diameter, sincethe process of electrical discharge hole piercing according to theadopted technique will be accompanied by a continuously increasingconsumption of electric power supplied to the electrode, while a majorpart of electric power will be spent for side spark erosion arisingbetween the face of the hole being pierced and the bare electrodethroughout its whole length equal to the length of hole being machined.Moreover, no working liquid recirculation in the machining zone leads toconstant clogging of the spark gap with the products of the sparkerosion process. Eventually, the hole piercing process ceases when thedepth of the hole being machined gets equal to about 8 or 10 electrodediameters.

SUMMARY OF THE INVENTION

The invention has for its object to provide a device for hole piercingin current-conducting materials, featuring such a structural design of acontrivance for near compensation of a tool electrode, which would makeit possible to increase production rate and accuracy of machining longand extralong holes.

The aforesaid object is accompanied due to the fact that in a device forelectrical discharge piercing of holes in components, comprising a toolelectrode mounted in an electrode holder and shaped as a rod which isenclosed in a sheath with a clearance to the latter and somewhatprojecting from said sheath, and a contrivance for traversing, lockingand wear compensation of the tool electrode, according to the invention,the contrivance for traversing, locking and wear compensation of thetool electrode incorporates a driving carriage and a driven carriage,both being machanically interlinked and set in parallel to each other,each of said carriages carrying a clamp adapted for alternately lockingthe rod-shaped tool electrode in position, said clamp being made as amovable jaw and a fixed jaw, while the housing of each of the carriageshas through centrealigning holes situated at the level of the gapbetween the movable and fixed clamp jaws and arranged coaxially with thechannel in a pipe union fixed in place in the bottom portion of theelectrode holder, the rod of the tool electrode being axiallytraversable in said channel, the sheath of the tool electrode being heldfast on the electrode holder pipe union coaxially with the rodshapedelectrode, whereas the electrode holder is fixed in place on the drivingcarriage.

It is expedient that the electrode holder be a hollow structure and thatits interior space be in communication with the channel accommodatingthe rod-shaped electrode, thus providing for supply of the working fluidthrough the gap between the sheath andthe rod-shaped electrode to thezone of workpiece machining.

The herein-proposed invention is instrumental in piercing long andextra-long holes in any current-conducting materials, the holes producedfeaturing high precision and high quality of surface finish.

BRIEF DESCRIPTION OF THE DRAWINGS

In what follows the herein-proposed invention will be illustrated by wayof some exemplary embodiments thereof to be read with reference to theaccompanying drawings, wherein:

FIGS. 1 and 1a are a schematic view of a device for electrical dischargepiercing of holes in current-conducting materials, according to theinvention;

FIG. 2 is a view of the drive of a mechanism of the arrestingcontrivance of the stop;

FIG. 3 is a longitudinal sectional view of the electrode holder;

FIG. 4 is a cross sectional view of the bath tub of the device; and

FIG. 5 is a sectional view of the carriage guideway.

BEST MODE OF CARRYING OUT THE INVENTION

The device for electrical discharge piercing of holes, according to theinvention, comprises a bath 1 (FIG. 1), a shaft 2 supported on bearings3 being mounted in the wall of said bath, said shaft being adapted for aworkpiece 4 to hold. The shaft 2 is connected to an electric motor 7through a pulley 5 and a belt transmission 6 so as to transmit rotationto the workpiece 4. A spacer sleeve 8 is set on the extension of theshaft 2 protruding beyond the limits of the bath 1, said sleeve beinginterposed between the pulley 5 and the bearing 3 so as to be in contactwith a spring-loaded brush 9 fixed on a covar 10 which is made from adielectric and is situated on the outside wall of the bath 1. Anelectric potential is applied to the workpiece 4 through the brush 9,the spacer sleeve 8, and the shaft 2, said potential having a polarityopposite to the polarity of the tool electrode (which is positive inthis particular case). The device incorporates also a contrivance fortraversing, locking and wear compensation of the tool electrode, saidcontrivance being made as a driving carriage 11 and a driven carriage 12mechanically interlinked and set in parallel to each other.

The driving carriage 11 is traversable on a guideway 13 set parallel tothe axis of rotation of the shaft 2, with the aid of rollers, 14, 15,16, (FIG. 2) and 17 (FIG. 1). A hollow electrode holder 18 is fixed inplace on the housing of the driving carriage 11, working fluid beingforce-fed into the hollow electrode holder through a flexible pipe 19. Asleeve made integral with the electrode holder 18 is located in thebottom portion of the latter. A dielectric bush 20 (FIG. 3) ispress-fitted into said sleeve, while a union 22 is accommodated in thedielectric bush 20 coaxially therewith and is held in place with a screw21. A through channel 23 is made in the union 22 arranged lengthwise itslongitudinal axis and coaxially with the axis of rotation of the shaft2. A thrust nut 24 with a seal 25 is provided at one end of the union22, and a rod 26 of the tool electrode is passed through the channel 23,the thrust nut 24 and the seal 25.

A sheath 27 is clearance-fitted onto the rod 26 of the tool electrodeand is held to the union 22 by a nut 28, so that the working end of therod 26 of the tool electrode be extended from the sheath 27 (FIG. 1) fora certain preset length `a`. A clamp of the tool electrode rod 26 islocated on the housing of the driving carriage 11, said clamp being madeas a movable jaw 29 and a fixed jaw 30. A tierod 32 of anelectromagnetic drive 33 located on the driving carriage 11 is connectedto the movable current-conducting jaw 29 through a dielectric plate 31.The movable current-conducting jaw 29 is electrically connected to theelectric circuit of the power supply unit (omitted in the Drawing) andhas an opposite polarity (which is negative in this particular case)with respect to the polarity of the workpiece 4. The movable jaw 29snugly forces the tool electrode rod 26, by virtue of the tension of aspring 34, against the fixed jaw 30, thus reliably locking the toolelectrode rod 26 in position. The fixed jaw 30 is held to a dielectricplate 35 which is fixed by screws on the housing of the driving carriage11. A respective electric potential is supplied from the electriccircuit of the power supply unit to the fixed current-conducting jaw 30in the same way as to the movable jaw 29. The driving carriage 11 isassociated, through a flexible link reeved through sheaves, with a drive36 for traversing the driving carriage 11. The drive 36 is connected tothe power supply and control unit from which control signals (commands)are delivered, which control the feed traverse of the tool electrode andits compensation. A screw 37 fixed with a locknut 38 is located on thehousing of the driving carriage 11. Turned onto the extending end of thescrew 37 are pusher nuts 39, 40 arrested with locknuts 41, 42. Thepusher nuts 39, 40 are so positioned as to be between two limit switches43, which are held to the brackets of the housing of the driven carriage12. Thus, mechanical association is established between the drivingcarriage 11 and the driven carriage 12. The latter carriage istraversable on four rollers 44 which rest upon the edges of the bothlongitudinal walls of the bath 1, which edges serve as the guideways ofthe driven carriage 12. The guide edges of the longitudinal walls of thebath 1 are arranged parallel to the guideway 13. Another clamp of thetool electrode rod 26 is situated on the housing of the driven carriage12, made as a movable jaw 45 and a fixed jaw 46. A tierod 48 of anelectromagnetic drive 49 located on the housing of the driven carriage12 is connected to the movable current-conducting jaw 45 through adielectric plate 47. The movable jaw 45 is electrically connected to theelectric circuit of the power supply unit (omitted in the Drawing) andhas an opposite polarity (which is negative in this particular case)with respect to the polarity of the workpiece 4 (FIG. 1). The movablejaw 45 is forced out, by virtue of the tension of a spring 50, of thefixed jaw 46, which is set over a dielectric plate 51 made fast on thehousing of the driven carriage 12 with two screws. A respective electricpotential is supplied from the electric circuit of the power supply unitto the fixed current-conducting jaw 46 in the same way as to the movablejaw 45. The housings of the carriages 11 and 12 have each a throughcentre-aligning hole situated at the level of the gap between themovable and fixed clamp jaws and arranged coaxially with the channel 23(FIG. 3).

Secured on the housing of the driven carriage 12 are brackets carryingbrake shoes 52 (FIG. 4), which are engageable with screws 53 arrangedeccentrically with respect to the axis of their rotation (with an amount`e` of eccentricity).

The screw 53 receives rotation from an electromagnetic drive 54 (FIG. 2)held to the tub of the bath 1. The drive 54 actuates the screw 53through a link 55 and a bell-crank 56. A spring 57 made fast, with oneof its ends on the bell-crank 56 and with the other end, on the rub ofthe bath 1, serves to return the screw 53 to the initial position. Theelectromagnetic drive 54 is mounted on a bracket 58 held to the tub ofthe bath 1 by screws. To provide electric insulation of the toolelectrode rod 26 (FIG. 1) whose end extends beyond the bath 1, from thetub of said bath, the rod 26 is accommodated in a dielectric bush 59press-fitted in the wall of the bath 1.

Whenever it becomes necessary the construction of the device beingdisclosed herein may provide rotation of the tool electrode rod 26 whichextends the processing capabilities of the device. This is attained dueto the provision of attachments 60, 61 which are held to the drivingcarriage 11 and to the driven carriage 12, respectively. Located at thebottom of the bath tub is a feeder for a traversable rest 62, whichserves for precision centre-aligning of the tool electrode in the courseof electrical discharge piercing of a hole in the workpiece 4. The rest62 is traversable along the axis of rotation of the shaft 2 from aspecial drive. Limit switches 64 (FIG. 5) are secured on the guideways13 through special clamps 63, said switches being longitudinallytraversable along the guideways 13 and fixable in a selected positionwith the aid of a stop screw 65. A drain port 66 is provided in thebottom of the bath 1 (FIG. 1) to discharge the working liquid therefrom.

The device for electrical discharge piercing of holes operates asfollows.

The workpiece 4 (FIG. 1) to be machined is clamped in, e.g., a chuck andset on the shaft 2. Depending on the amount of the workpiece portionextending from the chuck and on the length of hole to be pierced, thereis selected the initial position of the driving carriage 11 and thedriven carriage 12, as well as of the rest 62. Accordingly, there isselected the position of the limit switch 64 which restricts the travelof the driving carriage 11 and the driven carriage 12 in their leftmostposition. A drive 36 is provided to actuate the carriages 11 and 12 toassume their initial position. The rest 62 is set to the initialposition by engaging its special drive. The tool electrode rod 26preselected so as to suit the length of the hole to be pierced, is drawnthrough the dielectric bush 59, the through centre-aligning hole andfurther on through the opened jaws 45, 46 of the clamp for the toolelectrode rod 26 on the driven carriage 12. Next in order to pass thetool electrode rod 26 between the jaws 29, 30 of the clamp on thedriving carriage 11, the electromagnetic drive 33 is engaged by means ofa special switch (omitted in the Drawing). Once the drive 33 has causedthe movable jaw 29 to lift, the tool electrode rod 26 is drawn throughthe thus-established gap between the jaws 29 and 30. Then the rod 26 ispassed through the centering joles and gets into the channel 23 (FIG.3)of the union 22, whereupon it is brought outside the latter for arequired length. Thereupon the sheath 27 preselected so as to suit thesize of the hole to be pierced is fitted over the portion of the toolelectrode rod 26 extending beyond the union 22 and is locked on thelatter by a nut 28. The inside diameter of the sheath 27 should be soselected as to provide a required gap between the tool electrode rod 26and the inner suraces of the sheath 27. Once the sheath 27 has beensecured on the electrode holder 18, whenever necessary, the toolelectrode rod 26 is displaced in order to provide the amount of a presetextension `a` of its working end (FIG. 1) from the sheath 27. This done,the electromagnetic drive 33 of the driving carriage 11 is disengaged,with the result that the movable jaw 29 actuated by the spring 34,forces the tool electrode rod 26 against the fixed jaw 30 of the toolelectrode clamp, thereby fixing the tool electrode rod 26 in a requiredposition on the driving carriage 11. Then the driving carriage 11 ismade to traverse in order to fit the end of the tool electrode rod 26into the centre-aligning hole of the rest 62. Next the electric motor 7is started to impart rotation, via the belt transmission 6, to thepulley 5 and hence to the shaft 2 carrying the workpiece 4. The workingliquid is pressure-fed through the flexible pipe 19 into the interiorspace of the electrode holder 18, when the working liquid is passedthrough a special port and a plug of the union 22 into the channel 23and further on to the gap between the tool electrode rod 26 and theinner surface of the sheath 27. Thereupon the working liquid flowsbeyond the sheath 27 of the tool electrode to pass over the working endof the tool electrode rod 26 extending from the sheath 27.

Having made sure that the working liquid is fed properly to the zone ofmachining the workpiece 4 one should engage the power supply and controlunit (omitted in the Drawing), with the result that a voltage is appliedto the workpiece 4 through the brush 9, and to the tool electrode rod 26through the jaws 29, 30 of the clamp for the tool electrode jaw 26 ofthe driving carriage 11. Concurrently the driving carriage 11 along withthe tool electrode is actuated by the feed drive 36 to move towards theworkpiece 4. As soon as the spark (electrode) gap (that is, the gapbetween the working end of the tool electrode rod 26 and the surface ofthe workpiece 4) reaches a required amount, a spark discharges betweenthe tool electrode and the workpiece 4, which is indicative of abeginning of the electrical discharge process. Simultaneously with thetool electrode traverse the rest 62 is moved from its special drive.

While advancing the driving carriage 11 rests, through the pusher nut39, against the limit switch 43 and starts pulling the driven carriage12 behind. Having undone the locknut 38, one can alter the spacingbetween the carriages 11 and 12 by turning the screw 37 in or out so asto suit the diameter of the tool electrode rod 26. In a certain span oftime within which the process of electrical discharge piercing of a holeoccurs, necessity arises for restoring a preset length `a` of overhangof the working end of the rod 26 from the sheath 27 of the toolelectrode, since the projecting portion of the tool electrode rod 26becomes wornout, i.e., the consumable portion of the tool electrode isto be compensated for. The process of compensation occurs in thefollowing sequence. A control signal is delivered from the power supplyand control unit for the drive 36 to disengage and hence to stoptraversing of the driving carriage 12. Then the electromagnetic drives33 and 49 come into action so that the tierod 48, while overcoming theforce of the spring 50, presses the movable jaw 45 against the toolelectrode rod 26, thus locking it in the clamp of the driven carriage12. At the same time the bell-cranks 56 are actuated to turn, by theenergized electromagnetic drive 54 (FIG. 2) through the link 55, andwhile overcoming the tension of the spring 57, they rotate the screws 53(FIG. 4) which get engaged, by virtue of the eccentricity `e`, with thebrake shoes 52 provided with a screw thread. Thus, the driven carriage12 is locked in place with respect to the bath 1. Then the tierod 32rises while overcoming the tension of the spring 34, thus opening thejaws 29, 30 of the clamp of the driving carriage 12, whereby the toolelectrode rod 26 is released from locking in position.

Thereupon the drive 36 comes into action to impart backward motion tothe driving carriage 11 along with the tool electrode sheath 27, i.e.,in the direction opposite to that of the tool electrode feed traverse.The driving carriage 11 will perform backward motion until the pushernut 40 rests against the limit switch 43, while the tool electrodesheath 27 is displaced backward with respect to the immovable toolelectrode rod 26, since at a given instant of time the latter is lockedin place on the arrested driven carriage 12. As a result, the overhunglength of tool electrode rod 26 extending from the sheath 27 isincreased, thus compensating for the consumed amount of the rod endprojecting from the sheath 27. In this case the electrical dischargemachining process proceeds incessantly. The length of thecompensated-for consumable portion of the tool electrode is selected byappropriately adjusting the gap between the pusher nuts 39, 40 on thescrew 37 and the limit switches 43. As soon as the limit switch 43 isacted upon by the pusher nut 40 the drive 36 is disengaged, i.e., thebackward motion of the driving carriage 11 ceases, and the drivingcarriage 11 along with the tool electrode sheath 27 stops.

Just after this the electromagnetic drive 33 is deenergized and themovable jaw 29 of the clamp of the driving carriage 11 is actuated bythe spring 34 to move down, and the tool electrode rod 26 happens to betightly gripped between the jaws 29 and 30.

Then the electromagnetic drive 49 is engaged, and the movable jaw 45 ofthe clamp of the driven carriage 12 moves up, thus releasing the toolelectrode rod 26 from locking in position.

The feed drive 36 of the driving carriage 11 is engaged and the carriage11 starts traversing along with the tool electrode towards the workpiece4 at a rate of working feed of the electrical discharge machiningprocess.

As soon as the pusher nut 39 rests against the limit switch 43 locatedin the left-handportion of the driven carriage 12, the limit switchoperates to deenergize the electromagnetic drive 54 (FIG. 2). The screws33 (FIG. 1) are returned to the initial position under the action of thespring 57 and of the bell-crank 56 and are disengaged from the brakeshoes 52 (FIG. 2). The driven carriage 12 becomes unlocked and startsadvancing in the wake of the driving carriage 11. Thus, the electricaldischarge machining process continues. The entire procedure describedabove is repeated as frequently as required.

Once the preset depth (length) of hole in the workpiece 4 has beenattained, the driving carriage 11 rests against the limit switch 64 (onthe side of the workpiece being machined) and, after the limit switchhas come into action, the traverse drive 36 of the driving carriage 11is disengaged, and the carriage 11 stops. The tool electrode and theworkpiece 4 are deenergized, while the drive 36 and the special drive ofthe rest 62 are engaged, whereupon both of the carriages 11 and 12 andthe rest 62 are rapidly returned to the initial position until they restagainst the limit switches 64 and stop.

Whenever use is made of a tool electrode with the rotatable rod 26, theshaft 2 carrying the workpiece 4 is arrested with a stop screw. Theclamps of the both carriages 11 and 12 are completely disconnected fromthe power supply and control unit, whereupon the attachments 60 and 61are connected to the power supply and control unit instead. Theattachment 60 incorporates a rotation drive of the tool electrode rod 26and a clamp of the tool electrode rod 26 with a current lead to therotatable tool electrode rod 26. The attachment 61 incorporates a rotaryclamp of the tool electrode rod 26 with a current lead to the toolelectrode rod 26.

The aforementioned embodiment of the device operates in quite the sameway as in the case of the nonrotatably tool electrode rod 26 with theexception that the rotation drive of the shaft 2 is inoperative. Theelectrical discharge machining process proceeds incessantly in this caseas well at the instant when the tool electrode is compensated for wear.

The present invention provides for machining of long and extralong holesin any current-conducting materials having a diameter of from 0.5 to 3mm with high precision and quality of surface finish of the hole faces.This is favoured by the provision of a contrivance for wear compensationof the consumable portion of the tool electrode rod, whereby theoverhang length of the consumable portion of the tool electrode rodextending from the sheath, is maintained constant in the course of theelectrical discharge hole piercing, while the latter proceedsuninterruptedly at the instant when the consumable portion of the toolelectrode rod is being compensated for.

The device is conveniently insertable in an automatic control system ofthe electrical discharge hole piercing process, which ensures highprecision of machining and high quality of surface finish, safety inattending to the device, and does not require high skill from theattending personnel.

Industrial Applicability

The device for electrical discharge piercing of long and extralong holesin any current-conducting materials can find extensive application inthe mechanical engineering and machine-tool building industries, as wellas in the metal machining practice.

What is claimed is:
 1. A device for electrical discharge piercing ofholes in components, comprising a tool electrode mounted in an electrodeholder (18) and shaped as a rod (26) enclosed, with a clearance, in asheath (27) and somewhat projecting therefrom, and a contrivance fortraversing, locking and wear compensation of the tool electrode,characterized in that the contrivance for traversing, locking and wearcompensation of the tool electrode incorporates a driving carriage (11)and a driven carriage 12, which are mechanically interlinked and set inparallel to each other, each of the carriages carrying a clamp adaptedfor alternately locking the rod (26) of the tool electrode in position,the clamps being made as a movable jaw (29,30) and a fixed jaw (45,46),while the housing of each of the driving and driven carriages (11,12)has through centre-aligning holes situated at the level of the gapbetween the movable jaw (29,30) and the fixed jaw (45,46) and arrangedcoaxially with a channel (23) made in a union (22) fixed in place in thebottom portion of the electrode holder (18), the rod (26) of the toolelectrode being axially traversable in the channel (23), the sheath (27)of the tool electrode being fixed in position on the union (22) of theelectrode holder (18) coaxially with the rod (26) of the tool electrode,whereas the electrode holder (18) is fixed in place on the drivingcarriage (11).
 2. A device is as claimed in claim 1, characterized inthat the electrode holder (18) is a hollow structure and the interiorspace thereof is in communication with the channel (23) whichaccommodates the rod (26), thus providing for supply of the workingfluid through the gap between the sheath (27) and the rod (26) to thezone of machining of the workpiece (4).